By repressing expression of the myoB gene under the regulatable AlcA promoter, we also demonstrate that actin rings still form at septation sites even under condtions of reduced levels of wild type MyoB, but ring constriction and septum formation is prevented. In Movie S3 (below), actin is imaged in a myoB-repressed cell using an in vivo actin-binding Lifeact::mRFP construct. Note that actin rings form, but they fail to organize well and eventually disassemble without contracting.
Conversely, when filamentous actin is inhibited by latrunculin, MyoB rings fail to form.
We therefore conclude that actin does not require normal levels of myosin for basic actin ring assembly, but normal levels of myosin are required for actin ring contraction. Myosin, however, requires the presence of actin in order to participate in contractile ring formation.
Hill, T. W, L. Jackson-Hayes, X. Wang, and B. L. Hoge. 2015. "A mutation in the converter subdomain of Aspergillus nidulans MyoB blocks constriction of the actomyosin ring in cytokinesis." Fungal Genetics & Biology 75: 72-83.